Preparation of cellulose esters



Patented July 15, 1952 PREPARATION OF C ELLULOSE ESTERS George W. Seymour, Maplewood, Blanche B.

-White, Summit, and Mark Plunguian, Gillette, v N. J., assignors to Celanese Corporation of America, a corporation of Delaware Drawing.

This invention relates to the preparation of of organic acid esters of cellulose from said pretreated cellulosic material.

In the production of organic acide'sters of cellulose, cellulosic material is usually esterrified with an organic acid anhydride in'the presence 01 an ,esterification catalyst and an organic acid which acts as a solvent for the cellulose ester. It has been customary to pretreat the cellulosic material before esterification to render said material more reactive duringthe esterification step. Various expedients have been proposed to carry out such pretreatment; In; some instances-the cellulosic material is pretreated with alkaline swelling agents, and in other instances the cellulosic material is pretreated with lower aliphatic acids in either liquid or vapor form. These pretreatments have'been of some value in increasing the reactivity of the cellulosic material, but even when such pretreatments are employed the time necessary for esterification has ranged from 2 to '7 hours, or more. Moreover, the time necessary to carry out the pretreatment has ranged from 2 to 5 hours, or more, so that the total time necessary for pre-' treatment and esterification has ranged from 4 to 12 hours, or more. As a result, it has hitherto been necessary to carry out'both the pretreatment and the esteriflcati'on on a' batch basis requiring a large number of individual reactors, thus involving a large capital'outlay.

It is an important object of this invention to provide a process for preparing organic acid esters of cellulose which will be free from the foregoing and other disadvantages of the processes hitherto employed for this purpose, and which will be especially simple and eflicient in operation. v

A further object of this invention is the provision of a process for preparing organic acid esters of cellulose which may be carried out on 'a continuous basis.

Another object of this invention is to provide a process for preparing organic acid esters of cellulose in whichcellulosic material is subjected to a two-stage pretreatment prior to esterification. I v

A still further object of this invention is to provide a two-stage process for pretreating celhllosic material.

Other objects of this invention will be appar- Application November 2, 1948, Serial No. 58,028

14 .Claims. (Cl. 260229) 2 ent from the following detailed description and claims.' V

' In its broadest aspect this invention comprises a process for the preparation of organic acid esters of cellulose from cellulosic material, in which the cellulosic material is subjected to a two-stage pretreatment prior to esterification. In the first stage of the pretreatment the cellulosic material is treated with a lower aliphatic acid, and in the second stage the cellulosic material is treated for a period not exceeding about 15 minutes with an additional quantity of a lower aliphatic acid containing substantially all, namely from about 5 to 15 parts by weight of the esterification catalyst which will be. employed during-the subsequent esterification for each 100 parts by weight of the cellulosic material. Thereafter, an organic acid anhydride is added to the mixture formed during the pretreatment and esterification proceeds rapidly to completion. The lower aliphatic acidemployed during the pretreatment acts as a solvent for the cellulose ester, forming a clear dope of satisfactoryviseosity. v

During both stages of the pretreatmentthe pretreating agents must be so distributed as to reach all of the cellulosic material. Ifany of the cellulosic material is. left untreated, it will not be esterified during the esterification period, producing a cloudy dope containing unesterified particles. I l While our invention will be described .in connection with the preparation of cellulose acetate, it is to be understood that it'is also applicable to the preparation'of other organic acid esters of cellulose such as the simple and mixed lower aliphatic acid esters including/cellulose propionate, cellulose butyrate, cellulose aeetatepropionate and cellulose acetate butyratel In addition, it is'applicable'to the'preparationof such organic acid esters of cellulose as cellulose acetate stearate, as well as to the benzoic acid ester of cellulose.

The process of the present invention maybe applied to any suitable cellulosic material, such as, cotton linters, or wood pulps or the like from which the lignin, pentosan, resins and similar constituentshavebeen removed, as, for example, purified. sulfite pulp, sulfate pulp or soda pulp. In addition, there may be employed cellulosic material derived from other j sources such as bamboo, reeds, jute, esparto, staws such as, linseed and 'hempseed straw, grasses, peanut hulls, bark, etc. The cellulosic material may be pretreated in the form of a mass 'teriaL/ m I; :11

In the pretreating processeslhithertol proposed nospecialcare has been needed to: insure that ployed during the first pretreating stage in concentrations rangingwfrom about 8.0 to 100% by we ght and quantities ranging. from about of loose fibers, or ln the treating periods have ranged from two hours upwardly. However, if the high concentration of catalyst employed in the second pretreating stage losic material; as a result of which the cellulosic to 75, parts by weight for each 100 'parts by 7 weight of the cellulosic material. When the proportion of lower aliphatic acid is either larger or smaller than the preferred range, it has been found that the reactivity of the pretreated cellulosic material and the clarity of. the final dope decreases rapidly. If desired, up to about 2 par-ts by weight of an esterification catalyst may-be added to the lower aliphatic acid for each 100 parts by Weight of the cellulosic material; [The time of pretreatment during the first stage is not critical and, depending upon the efficiency with which the pretreating agent is distributed, may 7 range from 2 seconds, or evenless, to any desired value. Although shorter pretreating periodsduring this stage are preferred as being most -eificientfin the use or equipment and personnel, longer pretreating periods are not-objectionable smce they do not affect unfavorably the propertiesof the pretreated cellulosic-material. I V

I After thefirst vpretreating stage, it may be -desirable, in cert'ain instances, to permit'the cellulosic material tostandfor at least about 2 min .u't'es before continuing with the pretreatment. This allows. thelower .aliphatic acid t e-diffuse through the mass or. thickness. of. cellulosic material and into the cellulose. fibers, particularly when the cellulosic material is, in the form .of a sheenstrip or board. Ina continuous process, thenecessaryintervallpf time may he -obtained byfapplying, the pretreating. agents at spaced points. and moving the cellulosic materialirom onepointto the other at a givenspeed. W

1 lh'efirst pretreating; stagelis followed,, the ls econdpretreating stage, by thetre'atinent-of the cellulosic material for a period 'not. exceeding about 15 minutes with an additional quantityof.

loweraliphatic acid containing substantially all,' .namelyf from aboutfi to.1 5 partsbyweightof thejesterification catalyst for each. 100 parts by weight of the cellulosic material The; lower aliphatic acid is preferably employed inconcentrations rangingffrozn about 95 to 100% by weight and in quantities ranging from about 209, 1101600 parts by weight for each 100v parts by weight-of the jcelhrlosic materia1.

a Variousesterification catalysts may be em.- plo-ydduring the-pretreatment in accordance with the process of :this invention} butsulfuric acidi-isfpreferred for-this purpose since it produces a'cellulosic. material having the hig=he st reactiyitytirntheshortest.period of time. ln additiOn tof sulfuric acid, the-esterification catalyst may-also, be phosphoric acid, ,zinc. chloride or any 7 othen'ofi. the. catalysts that are Welljknowrr'for this-purpose;

Both stages of the pretreatment-may be carr-ied out at room temperature or. at temperatures aboveor below room temperature. I-Iowevenpretreatment at temperatures above or below room temperature issomewhat. less desirable owing to the necessity for artificiallyheating or cooling the pretreating. agents and the cellulosic maailior thecellulosic' material willcome into an;

material would no longer be suitable for the production of organic acid esters of cellulose. Accordingly, the time of pretreatment during the second stage which may be as low as about 2 seconds, or less, must be limited to not more than about 15 minutes. It is, therefore, particularly important that during this extremely short pretreating period, the pretreating agent be distributed soas to reach all of the cellulosic material to. avoid the production of a cloudy dope containing unesterified particles.

Although 'it has previously been suggested that a loose mass of cellulosic fibers be stirred continuously or intermittently to distribute the pretreatingag ent .d-uringpretreatment, the degree of sti-rring has beenrelatively slight since the pretreatingperiods have. ranged from two. hours upwardly.. During; this. extended period Tot time pedientsmay be employed to achievethisithorough distribution. Forexample, .whena loose mass, of cellulosic fibers; is, employed. theicelliilosicfibers and the pretreating agentmay he in troduced into a. stirrer 'equippedi with. paddle wheels operating. at speeds ranging from about 50. tov 1200 R. P. M., or more, for a ,suitableTperiod of.time. When employing equipment oi this type. it isimportant. toLavoid the presence or dead spotsin which a. portion ot the cellulosic-fibers may. remain during. the. pretreating: period: without coming into contact; withfthe pretreatingl agent. The cellulosic fibers and. the pretreating; agent may also be introduced into a-pulp-beater or a pulp refiner of. the" type employedin paper manufacture which, will fibrillate the cellulosic fibers during pretreatment as islmore full-y dis-s closed in our copending application, Serial Numwber 47,324,,filed September -1, 19.48;, The sameaor; diflerent types of apparatus may be employed; to; distribute the pretreating agents over the cellu-- losic material duringthetwostages of the p118?" treatment. If; desired, an interval of about 2 minutes, or. more, may be permitted to elapse between. the. first and. second stages-of the; pretreatment to allow the pretreating agentto dif fuse throughthe cellulosic-material.. L

On the other hand,when' a sheet, strip;or board of cellulosic material is employed, the pretreating agents may be applied to one-orboth-surfaces of said sheet, strip-or board:bysprayingiidipping; roller coating orsother;similarmanner;v It iSIiIlI- portant, in this case, to allow a time: mtervalita elapse-between thefirst ands'econd'stagesof pretreatment to permit the ,pretreatingagent employed during theifirst s-tagertof diffuse; through the cellulosic material-..- After the second pre;-; treating agent is applied, the sheet; strip or board:

of cellalosiematerial be feddireetlyi into. an

acetylizer: However, if desired, the.sheet,; strip'or board may first be disintegrated in an agi-tatolgjby" means ora rapidly rotating. wire bruslr or many other manner before being-fed to the acetylizer; To' test the adequacy with which a given procedure distributes the*prei'ireat'ing agents through the' cellulosic material during either of. the two pretreatin'g stages; a small amount'of a-dye 'such stirred for 5 minutes in a mixer equipped.- with horizontal paddle wheels operating at 50 R'. R;

An additional 285 parts by'weight of acetic acid as amethyst violet maybe added' to. th e 'pleir- 1 treating agent during atrial rum -When-thecel may be employed at room temperature, or above.

Esterification proceeds rapidly and is completed in from about 2 J to 10 minutes, or less. "The lower aliphatic acid acts as a solvent for the cellulose ester yieldinga clear dope of satisfactory viscosity? After .esteriiicati'oniscompleted; the cellulose ester may be ripened, precipitated by theadditionof water to thedope, stabilized, if desired, anddrieds Wh'enHthe process 'of'this invention is employed, it, is possible t'o' carry'out the pretreatment and esteifiilcatio'n on acontinuous rather than .on a batch" basis, since the time required for both; treatments is sufiiciently short "to render a con-P tin'uous process feasible;

The following examples are given as illustrative of this inventions 7 I I fEx ample 'I it H A mixture of 100 parts by weight r cotton um;

ers andj 35 partsbyflweight of glacial acetic acid are stirred .io'r B'minutesinia cylindrical mixer equipped with horizontal paddle wheels operating at 50 R. P. M. An additional 286 parts by weight of glacial acetic acid containing 14 parts by weight of 100%.suliuric. acid are introducedinto the mixer and stirring continued for .2 'rninutes. The cotton linters and pretreating agents a're'introduced into 215'partsby weight of acetic anhydride precooled to 0 C. .Esteriflcatlon proceeds smoothly withthe temperaturez'ris'ing to 65?".Cfat,

the endof 3 minutes, at whichti'me there'yi's'i obtamed a clear dope of satisfactory viscosity Excntple II-jli'f A mixture of 100 parts by 6. 1"" factory viscosity at the end of two minutes. a weightof cott n linters, 35 parts by weight bf 'glacialacetic acidiand' 0.5" part by weight of 100% sulfuric acid are stirred for 5 minutes in a cylindrical mixer equipped with horizontal paddle wheels operating at 50 R. P. M. of glacial acetic acid containing 13.5 parts by weight of 100% sulfuric acid are introduced into the mixer and stirring continued for 2 minutes. The cotton linters and pretreating agents are introduced into2l5 parts by weight of acetic dride'precooled to 0 C.. Esterification proceeds smoothly with the temperature rising to C. at the endof three minutes, at which time there'is obtained a clear dope of satisfactory viscosity.

Example III A mixture of 100 partsby weight of cotton lintanhy-j.

An additional 286 parts by weight I "and 13.5 parts=by weight of 100% sulfuric acid are introduced intothe cotton linters andithe lwholerun between the plates of a conventional pulp grinder operating at 3600 RP; Mtwith the clearance between the plates set at 0.10 vinchto fibrillate the cellulosic material. The mixture of fibrillat'ed cellulosic material and pretreating agents is introduced into-215 parts by weight of acetic anhydride precooled to .14 C. Esterification proceeds smoothly-"with temperature rising to 51 C. at the end or 7 minutesjat which time there is obtained a: clear dope of satisfactory viscosity. 1 1 ".J' m P A mixture of 100 parts by weightof cotton; ers and 35 parts-by weight of glacialacetic acid. are stirred for 10 seconds in a cylinder mixer equipped with; paddlewheels operating at l20 R. P. M. The mixture is allowed to stand for- 5 minutes, whereupon anadditional 285 parts byweightjof glacial acetic acid containing l iparts by weight of 100% sulfuric acid are introduced into the mixer and stirring continuedforfiqseconds. The cotton linters and pretreating agents are introduced into'230 parts by weight of acetic anhydride precooled to 10 C. Esterification proceeds smoothly yielding a clear dope of satisfactory viscosity at the endof three minutes.

Example V Strips of cotton sheet are passed continuously througha spray chamber containingtwo, spaced I sets of spray nozzles arranged in such a manner that, the sprayed liquid impinges equally "and uniformly on both sides'of' the sheet. vFroinLthe first set of spray nozzles, glacial aceticfacidis sprayed onto the sheets in an amount equal to parts by weight for each parts by weightof cellulosic material. The sheets travelfor five minutesbefore reaching the second set of spray,

nozzlesffrom which a mixturecontaining9."

by'weight of glacial aceticacid and5% byw'eight. of 100% sulfuricacid s sprayed ontoithe sheets; in, an amount equal to ,300 parts by weight of, said mixture for each 100 parts by weight 'of cellulosic materiah Thereafter, thefsheets are.

and'the cellulosic material lS. eStelifid, in the manner outlined in Example. IV .v Esterificat'ion proceeds smoothly yielding a clear, dope of satis It is to be understood that the foregoing taileddescription. is, given merely by wayi' qf,

- to the cellulosic material containing the lower aliphatic acid of a further quantity of alower aliphatic acid containing all of anesterification catalystwhich will be employed during the subdisintegrated by means of a rotating metal brush r 'nit ithe diffnsion of theflower aliphatic acid sequentaes'terification of. the cellulosic ,material without displacing any of the lower aliphatic acid contained in the 'cellulosic material; and

esterif'ying' the pretreated "cellulosic material withinrza aperiod ofless than about" 15 minutes after the addition thereto of the furtherquantity of lower aliphatic acid containing the esterificae 1 -3; In a process for the preparationof organic I acid esters of cellulose from cellulosic material wherein the esterificationproceeds. rapidly, the steps *which" comprise subjecting *the cellulosic material to a multi-sta'ge pretreatment prior-to 'esterification, one- --s tage;' of said pretreatment comprising the addition to the cellulosic maren l-er alower' aliphatic acid and another stage 7 oi'sa idpretreatment comprising theraddition to the' celliilos'ic mater'ial'con'taining the lower ali-' phatie: acid 01 afurther quantity of a lower all phatic acid containing from about 5 to parts by weight'of anesterification catalyst for each IOOpartS by "weight of cellulosic materialwithout displacing any of the lower aliphatic acid ceata neam the cellulosic material; and esteriith Y pretreated cellulo'sic material within a period ofle'ssthan abowut 15jminutes after the additionthereto of the further quantity of lower c acid containing the esteri'ficationcata alipha lyst. V

- 4. Process according to, claim 3-, wherein there is. an interyalo' frat least about 2 minutes between the two stages of -thepretreatment to perthrougli the cellulosicm'aterial;

5 Process accordin acid is employed as" the pretreating acid. 7 a a 6'. j Process a according to claim 3, wherein sul iuricj acid. is employed, asth'e esterification "cata employed during the following stage of; thepretreatmentQl v V 7 8;; Process according to claimf7, wherein acetic acid is employed as the ,pretreating acid, and sulfuric; acid is employed; a s the esterification l -FTC 1 we l.

ant-a process' f or"thejpreparation of cellulose acetate flom 'cellulosic material wherein the. esterification' proceeds rapidly; the steps which comprise subjecting" the cellulosic material "to a g to claimi3, wherein -acetie multi-stage pretreatment; prion to esterification,

one stage; of said pretreatment; comprisingthe addition to-the cellulosic materialaof a lower .-ali-. phaticf acidand; another; stage of said pretreat= ment-z=comprising ;the addition to. the -;cellul'osic material containing the-lower-aliphatic acid of a further. quantity ':of a lower:aliphatic'acidlcontaining fromgabout fi/v-to 1-5 partsybyweightgof an :esterification cataly t foneach 1010 parts weight of cellulosic- {material without displacing any; of; the lower;aliphatic ,acid contained in the cellulosic material, and esterifyingthe prertreated cellulosictmaterial' with aceti'c anhydride within .a period of lessthan about 15 minutes afterthe addition thereto of the further quantity of Blower aliphatic acid containing 7 the esterification catalyst; :5 a ;,=--I 1 :10; Process according to claim 9, whereinthere is an interval of at least about 2'minutesbetween. the two stages of -the pretreatment to permit the diffusion; of: :the lower aliphatic acid through the cellulosic:material. I "-11; Process -.according to 'claimw9,. wherein acetic acid is :employed as the pretreating acid. 12. Process according to-claim 9, wherein-s ll: furic acid is employed as the esterification: catalyst. A t p 13. Process accordingto claim 9;-wherein from about 15 to parts by weight of lower aliphatic acid for;each parts lay-weight of cellulosic material are employed during the first stage of thexpr'etrea'tment, andsf'rom about; ;200 :=to I606+ par-ts by. weight of lower aliphatic acid for ,each- 100: parts by weight :of cellulosic material are-ems 1 ployed during the following stage of thepre treatment. J

V 14. Process according to claim 9} whereinacetic acid is employed as the pretreating acid, a

and sulfuric acid is e tion-catalyst;= 7 l v 5 GEORGEQW? SEYMOURN";

I i -BLANCHEJB; WHI'I'EJ I MARK PLUNGUIA-Nn a v "Riz ERENcEs fo ITEo" The-renewin ref en s"ief l r n his ployed as the esterifica- Number: "=,.:-"Nanie Date 2,140,639 #Malm et -aL'W D'ec.-.20,- 1938- 2,206,288 Malm* f July 2, 1940 7 23153973 7 Malm -'Apr.1.6; 1943 2,400,361 Malmetal.' May 14, 1946 2,478,383 Fordyce et 'al. Aug. 9, 1949 2,478,425 F Richter 'et al. i; Aug; 9,: "1949 2,484,455 --Herdle Vet Eal: Oct.a11,- 1949 2,487,892 Richter et a1. Nov. .15, 1949 

9. IN A PORCESS FOR THE PREPARATION OF CELLULOSE ACETATE FROM CELLULOSIC MATERIAL WHEREIN THE ESTERIFICATION PROCEEDS RAPIDLY, THE STEPS WHICH COMPRISES SUBJECTING THE CELLULOSIC MATERIAL TO A MULTI-STAGE PRETREATMENT PRIOR TO ESTERIFICATION, ONE STAGE OF SAID PRETREATMENT COMPRISING THE ADDITION TO THE CELLULOSIC MATERIAL OF A LOWER ALIPHATIC ACID AND ANOTHER STAGE OF SAID PRETREATMENT COMPRISING THE ADDITION TO THE CELLULOSIC MATERIAL CONTAINING THE LOWER ALIPHATIC ACID CONA FURTHER QUANTITY OF A LOWER ALIPHATIC ACID CONTAINING FROM ABOUT 5 TO 15 PARTS BY WEIGHT OF AN ESTERIFICATION CATALYST FOR EACH 100 PARTS BY WEIGHT OF CELLULOSIC MATERIAL WITHOUT DISPLACING ANY OF THE LOWER ALIPHATIC ACID CONTAINED IN THE CELLULOSIC MATERIAL, AND ESTERIFYING THE PRETREATED CELLULOSIC MATERIAL WITH ACETIC ANHYDRIDE WITHOUT A PERIOD OF LESS THAN ABOUT 15 MINUTES AFTER THE ADDITION THERETO OF THE FURTHER QUANTITY OF LOWER ALIPHATIC ACID CONTAINING THE ESTERIFICATION CATALYST. 